1. Field of the Invention
This invention relates to power cable jointing, and more particularly to a re-enterable cable jointing connector and a re-enterable cable connection and isolation method.
2. Description of the Prior Art
Electrical utility companies use two basic types of cable for low voltage (.ltoreq.1,000 volts, .about.1,000 amps) installations: single core and multicore. Single core cables are now all polymer insulated, using materials such as polyvinylchloride or cross-linked polyethylene. Multicore cables are either polymer or paper insulated and are, in the majority of cases, either four-core or three-core with surrounding neutral/ground wires. Paper insulated multicore cables are all metal sheathed and oil-filled, some draining, some non-draining. Polymer insulated multicore cables may be armored with steel wires or tapes. Most low voltage installations today are three phase plus neutral and/or ground. In the majority of cases, neutral and ground are now combined. Moreover, in the United States at least, reference to a single core cable is often synonymous with single phase.
Three basic types of low voltage joints are incorporated into the electrical network: in-line joints, branch joints and service connections. In-line joints are made when two cables are connected together or when a cable is repaired after failure. A branch joint (T-joint) is a second main cable jointed into the first main cable. Usually the cables involved in a branch joint are similar in size and have similar current carrying capabilities. The third type of joint, service connections, are taps into the distribution network for small consumers, such as, homes, small factories or street lighting. The number of service cables which can be connected to the main cable via a service joint can presently be as high as six, but four is more typical. It is desirable for service connections to be re-entered to add additional services or to break and remake existing connections.
The evolution of power distribution technology has made jointing of non-compatible cables a major problem for the electrical industry which now mixes cable types extensively within distribution networks. Paper insulated cables are damaged by water ingress, and hence joints and terminations in such cables must be waterproof. Paper insulated cables jointed directly to polymer insulated cables are therefore susceptible to damage, such as from water inadvertantly channeling through the interstices of a multicore polymer insulated cable. The insulation of low voltage polymer insulated cables are not susceptible to moisture and the majority of such cables are not water blocked, however, the insulation may be damaged by contact with oil, such as from oil-filled paper insulated cables. Polymer insulated cables jointed directly to oil-filled paper insulated cables are thus susceptible to deterioration from contact with the oil.
The first cables were paper insulated and easily damaged by moisture ingress. The first joints consisted of metal half-shells filled with an insulation medium such as oil or bitumen. When the first multicore plastic insulated cables were introduced, the same basic approaches as were used with paper insulated cables were used again, except that the details were modified slightly. One such modification is the use of casting resin to replace bitumen and a simple vacuum-formed housing to replace the metal shell. As both types of cables became used on the same network, various compatibility problems resulted. One of the more widely used solutions to multicore cable jointing compatibility problems was the low voltage distribution pillar-pedestal, into which the cables were brought and interconnections made using a series of small bus bars, a miniature version of an indoor distribution board. Such a pedestal is typically ground mounted, about 30 inches high and 15 inches square and is often regarded by engineers as a poor alternative to underground jointing. The pedestal is not compact in size and is subject to problems such as condensation, flooding and vehicular damage.
Analogous approaches are used on single core cables. The most recent innovation in this area was a low voltage single phase multiple connector ("the octopus"), which is a small, insulated bus bar-type device which incorporates the ability for individual connection points to be accessed, a butt connection made and subsequently sealed without effecting other connections. Although compact, this device is limited to single phase use because it cannot be scaled up as a practical matter. Multicore cables are very rigid and their cores not easily bent. Three phase devices of this design would not be compact and would require four of five separate bus bars depending on whether neutral and ground were combined. The butt-style of connection would result in unacceptably long lengths of bare cores, which would pose an especial hazard if connections were made under tension. Cores of paper insulated cables would be especially susceptible to damage. Thus, no compact device exists which allows three phase and neutral joints to be made, sealed and subsequently re-entered without difficulty. No single compact device exists which allows jointing of size, configuration or material incompatible cables, especially those to be buried.